The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.
The use of indenters to obtain data indicative of mechanical properties of test specimens is known. U.S. Pat. No. 4,848,141 describes a method for continuously measuring the elastic response of a mechanical junction between an indenter and a sample material by applying a predetermined increasing direct force to said indenter to load the junction while applying a relatively small amplitude oscillatory force to the junction of sufficient magnitude to alternately load and unload the junction while continuously measuring both the amplitude and phase of the resulting displacement of the indenter relative to the applied oscillatory force as a continuous indication of the stiffness of the junction as the indenter is forced against the sample in a process of measuring various other mechanical properties of the sample material.
This patent further describes a method to measure stiffness (i.e., elastic and inelastic response) of a sample material in an indentation test system, wherein an indenter probe is forced into contact with the sample over a loading and unloading cycle, as the indentation process is carried out without interrupting the continuity of the process. This is made possible by superimposing a relatively high frequency AC signal source onto a DC signal used to drive a force generating means that applies the force to the indenter probe. The DC driving force is a very slow changing applied force. The AC driving force is sized such that a displacement amplitude (typically 10 Angstroms prior to contact) results. The force alternates at a selected frequency which can range from about 0.5 Hz to 1 MHz (megahertz) depending on the mechanics of the indenter probe assembly and the capabilities of the detection electronics. The alternating displacement component of the signal taken from a displacement gage is monitored by a frequency specific amplifier which also determines the phase of the signal relative to the applied AC drive signal. This provides a measure of the slope of the unloading versus displacement curve, i.e., continuous measurement of the elastic load/displacement response of the contact of the indenter with the sample. This approach yields the desired measurement of stiffness without significantly changing the DC force component acting on the junction. Using this method, a direct, virtually instantaneous and continuous measure of stiffness of the contact junction between the indenter and the sample is obtained as a function of the oscillatory displacement of the indenter probe tip during the process of loading and unloading the indenter and sample junction, i.e., as plastic deformation of the sample occurs.
Although the foregoing is useful in measuring stiffness of contact for a sample subjected to indentation, other specimens such as thin films have other material characteristics that need to be measured, but are unsuited for this type of testing. In particular, there is a need to measure or obtain relationships of properties of thin films such as but not limited to the elastic modulus and residue stress.